To call someone 'n***a'?

Yes, it sounds racist. It is an insulting name for black people. You should never use it.

AnonymousI've seen a lot of teenagers frequently use this word.

The only people who can use it with impunity are African-Americans. The context may be discussions of racial prejudice.
Otherwise, it is very very very rude and insulting.

Thank you again!
Sometimes I see teenagers call their close friends by that word and it seems like they have no intention to sound racist. Is that just because they're teenagers or what? hahaha

Like any insulting word, it may sometimes be used between friends with no offence intended or given, but it is a word that is generally regarded as deeply insulting. Even when used between friends, it could offend someone else who accidentally overheard it, or it could cause offence if someone who is not a member of the group says it after hearing the others use it.

AnonymousSometimes I see teenagers call their close friends by that word

Are these teenagers of African descent?

No, they are american asians.

It's not surprising that the entry for the word in Urban Dictionary (http://www.urbandictionary.com/) is very very long. Apparently it is being used among circles of close friends of any racial minority, not just African-Americans.